The present invention pertains to the detection of collisions and conditions of excessive force encountered between objects such as industrial robot arms and workpieces. In particular, the present invention relates to a piezoelectric load cell which finds advantageous utility when applied to a robot arm as a collision detector.
In the industrial use of robots for assembly and other applications, errors which may occur in location, part size or part shape can lead to collisions or force fit conditions which undesirably generate large and often excessive forces which can damage either the robot, part, workpiece, or all of them.
It is therefore necessary and desirable to detect such collision and force fit situations as may occur during the operation of assembly and other types of industrial robots, in order to prevent the occurrence of damage.
Various devices can be used to detect robot arm end effector collisions. For example, it is possible to provide tactile sensors of various types on the robot arm end effector. However, if a collision occurs between a part carried by the robot and another part or tool, as might occur during an assembly operation, the tactile sensor may not necessarily be involved and thus not be able to detect the collision the force of which therefore may go undetected.
It is known to install so-called "instrumented remote center compliances" (IRCC's) between the robot wrist and end effector for detecting compressive forces resulting from collisions, as well as lateral deflections from other causes such as misalignment. However, available IRCC's are relatively expensive, and provide much more information than is required merely for collision detection. As a result, a relatively large amount of time is required to transmit the data to the robot control, and to analyze the data to determine whether a potentially disastrous collision has occurred, or some other condition. Further, IRCC's are relatively large and heavy, which is disadvantageous to their use as collision detectors, in that the complexity, mass and bulk of the robot arm are undesirably increased thereby.
Known force measurement devices utilized in robotic machinery include the device disclosed in U.S. Pat. No. 3,952,880. In this patent there is disclosed a manipulator including an end effector, with a system of plural sensing units including photocells disposed between the wrist and end effector for sensing both the magnitude and direction of forces acting along three mutually orthogonal axes intersecting at the robot's wrist.
U.S. Pat. No. 4,648,784 discloses a drive protection device which is connectable between two portions of a driven operating device such as a robot manipulator, for protecting against collisions. If a collision occurs, the positions of ball elements spacing apart a pair of disks is changed, and resultant movement of the disks actuates a switch for causing disconnection of the drive.
Such known devices are, however, mechanically complex, and thus there remains a need for a simple, less complex device for detecting robot collisions.
In the present invention, there is provided a piezoelectric load cell robot collision detector which in a preferred embodiment senses compressive forces at the robot wrist. The invention utilizes a thin film of polyvinylidene fluoride (PVDF), which is a piezoelectric material. When stressed, PVDF film generates a difference in potential between its two surfaces. These surfaces are metallized for electrical contact with the film. The metallized PVDF film and a resilient pad are pre-compressed between two plate-like members, and this assembly is mounted between the robot wrist and end effector. If a collision affecting the end effector occurs, the collision forces further compress the PVDF film, generating a voltage pulse which is then amplified and can then be transmitted to the robot control for analysis.
The use of piezoelectric force sensors is well known in various arts. For example, U.S. Pat. No. 4,286,459 discloses a force sensor formed as a flexible piezoelectric strip which is stretched in tension and caused to oscillate, producing pulse output signals at a certain frequency, the sensor being used for measuring tension.
U.S. Pat. No. 4,634,917 discloses an active multi-layer piezoelectric tactile sensor utilizing a piezoelectric PVF2 sensing layer.
U.S. Pat. No. 4,555,953 discloses a composite, multifunctional tactile sensor utilizing a piezoelectric material such a PVF2 or PVDF.
U.S. Pat. No. 4,499,394 discloses an encapsulated piezoelectric plastic pressure sensor utilizing PVDF film for sensing compressive forces resulting from the foot pressure of an animal, and usable while the animal is under locomotion.
U.S. Pat. No. 4,488,873 discloses a piezoelectric occlusal force sensor utilizing a folded PVDF film structure for sensing the bite force exerted by a dental patient.
U.S. Pat. No. 4,348,142 discloses a control handle of the joystick type for permitting an operator to control a robot manipulator in six axes. The handle is supported by plural force-sensing bearings which in one embodiment consist of piezoelectric capsule sensors fed by a high frequency generator.
These known piezoelectric force sensors do not, however, lend themselves to use in detecting robot collisions. In particular, a robot collision detector must not only be capable of sensing collision forces but also, importantly, must provide structural integrity and rigidity to the robot arm as well.
In the piezoelectric collision detector according to the present invention, the use of the plate-like members provides necessary structural rigidity and integrity to the robot arm in which the detector is installed. One plate-like member is adapted for attachment to the robot wrist, while the other plate-like member is adapted for attachment to the robot end effector. Under the influence of forces resulting from, for example, obstruction of the end effector, the plate-like members transmit compressive forces to the piezoelectric film sandwiched therebetween. Means are provided for permitting relative yielding axial deflection to occur between the plate-like members while maintaining necessary structural alignment therebetween in order to maintain structural rigidity between the robot wrist and end effector.
It is therefore an object of the present invention to provide a piezoelectric load cell which is capable of effectively detecting the forces resulting from robot collisions and obstructions.
It is also an object of the present invention to provide a robot collision detector which is simple in construction and relatively inexpensive to manufacture.
It is further an object of the present invention to provide a robot collision detector which utilizes a piezoelectric film force sensor means.
It is still further an object of the present invention to provide a piezoelectric load cell which may be mounted between a robot wrist and end effector for detecting compressive forces therebetween.
It is yet another object of the present invention to provide a piezoelectric load cell robot collision detector which when mounted between a robot wrist and end effector provides for the detection of compressive forces acting therebetween while also maintaining structural alignment and integrity therebetween.